This invention relates to controlling the flow of molten material such as glass from a vessel. It is particularly applicable to an outlet from the bottom of a vertically elongated vessel.
Various means are known in the art for controlling the flow of molten material such as glass through a gravity fed drain. Many of these arrangements involve manipulating the viscosity of the material by variably heating or cooling the drain tube. An induction coil associated with a drain tube is a typical example. In some cases these approaches may be employed satisfactorily, but they posses certain drawbacks in the large scale melting of glass. The quantity of heat in a rapidly flowing stream of glass (e.g., on the order of hundreds or thousands of kilograms per hour) is so great that it is difficult to significantly influence the viscosity of the stream by heat transfer through walls of a drain tube. On the other hand, when adequate heat exchange is provided to effect control of the flow rate, the sensitivity of glass viscosity to temperature makes it difficult to finely modulate the flow rate. physical flow restricting means ("plungers") are well known in the art for controlling molten glass flow. A typical plunger arrangement involves structural elements within the melting vessel that interact with the upper end of a drain orifice. Such an arrangement would present problems in a vessel having a large height of molten glass in that rigidity of the plunger shaft would be difficult to maintain without excessive cooling.
In U.S. Pat. No. 4,600,426 (Schwenninger) there is provided an arrangement for metering the flow of molten material from the bottom of a deep vessel by mechanical throttle means engaging the exterior of an outlet tube. A bulbous member supported by a laterally extending arm constitutes the throttle means. The support arm has downwardly converging surfaces in order to promote a coherent flow of the molten material. That arrangement is capable of satisfactory performance, particularly when the throttle member is fabricated of or coated with platinum or alloys thereof. It is desirable, however, to use less costly materials such as molybdenum, which has excellent durability in contact with molten glass, but oxidizes rapidly when exposed to an oxygen-containing atmosphere at elevated temperatures. Therefore, the bulbous portion of the throttle member can be made of molybdenum since it is enveloped by molten glass, but the support arm cannot due to its exposure to the atmosphere. If the arm is preserved by cooling, it has been found that the molten glass stream does not wet the cooled element, and a long, open furrow is created in the stream below the support arm. As a result, a path is opened for attack of atmospheric oxygen on the molybdenum bulbous portion. Prior to the present invention, this dilemma has thwarted attempts to use less costly materials in devices of this type.